1. Field of Invention
This invention relates to optical signal processing, and more particularly to the use of fractional Fourier transform properties of lenses with traditional non-phase-shifting optical elements within traditional Fourier optical signal processing environments to realize, or closely approximate, arbitrary non-positive-definite transfer functions. The system and method herein can be applied to conventional lens-based optical image processing systems as well as to systems with other types of elements obeying Fractional Fourier optical models and as well to widely ranging environments such as integrated optics, optical computing systems, particle beam systems, radiation accelerators, and astronomical observation methods.
2. Discussion of the Related Art
A number of references are cited herein; these are provided in a numbered list at the end of the Detailed Description. These references are cited as needed through the text by reference number(s) enclosed in square brackets. Further, the cited disclosure contained within reference [1-19] is hereby incorporated by reference.
The Fourier transforming properties of simple lenses and related optical elements is well known and heavily used in a branch of engineering known as “Fourier Optics” [1, 2]. Classical Fourier Optics [1, 2, 3, 4] allows for flexible signal processing of images by (1) using lenses or other means to take a first two-dimensional Fourier transform of an optical wavefront, thus creating at a particular spatial location a “Fourier plane” wherein the amplitude distribution of an original two-dimensional optical image becomes the two-dimensional Fourier transform of itself, (2) using a translucent plate or similar means in this location to introduce an optical transfer function operation on the optical wavefront, and (3) using lenses or other means to take a second Fourier transform which, within possible scaling and orientation differences, amounts to the convolution of the impulse response corresponding to the optical transfer function with the original image. In this way images can be relatively easily and inexpensively lowpass-filtered (details softened) and highpass-filtered (details enhanced) as well as multitude of other possibilities. These multitudes of possibilities have, due to properties of materials and fabrication limitations in transcending them, been limited to transfer functions that mathematically are “positive-definite;” that is, those which affect only amplitude and do not introduce varying phase relationships.
The Fractional Fourier transform has been independently developed several times over the years [5, 7, 8, 9, 10, 14, 15] and is related to several other mathematical objects such as the Bargmann transform [8] and the Hermite semigroup [13]. As shown in [5], the most general form of optical properties of lenses and other related elements [1, 2, 3] can be transformed into a Fractional Fourier transform representation. This fact, too, has been apparently independently rediscovered some years later and worked on steadily ever since (see for example [6]) expanding the number of optical elements and situations covered. It is important to remark, however, that the lens modeling approach in the later long ongoing series of papers view the multiplicative-constant phase term in the true farm of the Fractional Fourier transform as a problem or annoyance and usually omit it from consideration; this is odd as, for example, it is relatively simple to take the expression for lenses from [2] and repeat the development in [5] based on the simplified expression in [1] and exactly account for this multiplicative-constant phase term.